1. Field of the Disclosure
The present disclosure relates generally to adaptive gain control of a transmitter, and more particularly, to setting a transmission gain in a transmitter of an electronic device to maintain a constant Error Vector Magnitude (EVM) as the load impedance varies.
2. Description of the Related Art
A radio transmitter for mobile applications typically includes a source (e.g., a power amplifier) connected to a load (e.g., an antenna) via at least one of a filter and a switch. The amplifier is designed to provide best performance on a nominal load impedance of (50+j0)Ω. However, the antenna is often exposed to conditions that disrupt its radiated electromagnetic field. Such conditions may include an object in close proximity to the antenna. These conditions cause the equivalent impedance of the load to diverge significantly from the nominal value.
In order to maximize power transfer from a source to a load in an electrical circuit, the load impedance must equal the complex conjugate of the source impedance. When the load impedance is not equal to the complex conjugate of the source impedance, the power transferred to the load is not optimal, which results in mismatch power loss.
Due to the intrinsic limitations in both voltage and current swing of the amplifier, load mismatch may also cause distortion of the transmitted signal. Distortion is often measured in terms of EVM, and causes a reduction in the Signal-to-Noise Ratio (SNR) at the receiver end. This distortion, and resulting reduced SNR, causes the received signal to be unintelligible, and may eventually cause the link to fail.
Traditionally, in dealing with antenna mismatch, an isolator is inserted between the power amplifier and the antenna. The purpose of the isolator is to prevent the power reflected from the mismatched load to enter back into the amplifier, which can cause distortion of the signal and potential damage to the amplifier. In addition to an increase in the Bill Of Materials (BOM), the insertion of the isolator also reduces the radiated output power, which is equivalent to the insertion loss of the isolator.
A more elaborate approach to overcome antenna mismatch is the insertion of a tunable matching network between the power amplifier and the antenna. This robust solution to maximize the output power requires tunable discrete components that tend to be bulky, expensive, and result in non-negligible insertion losses, even when driving a perfectly matched load. Further, such components are typically not available in Complimentary Metal-Oxide Semiconductor (CMOS) integrated technologies.
A number of analog adaptive techniques have also been developed to automatically adjust transmit gain based on power or voltage that are detected to go into the load. However, these techniques require dedicated analog circuitry and do not allow for a programmable EVM target.
These adaptive techniques rely on reducing the bias current of the amplifier, which reduces the gain and prevents signal clipping. However, reducing the bias current also reduces the linear dynamic range of the amplifier, which causes increased distortion and EVM. The increased distortion and EVM have become more of a concern as requirements for supporting high data rate modulations increase.